LM3886/Vox SS Amp Thread(w/ Samples!)

[RDV]

I'm going to build the SS Vox preamp suggested to me in Ampage. Here it is.

I'm going to also try the limiter circuit suggested by R.G. Here it is. I took the liberty of removing the other two input channels of the limiter.

These are going into my LM3886 practice amp I've built. My question is about supply voltage. I've got a seperate 12v supply that I use to run a cooling fan and whichever preamp I happen to be running. I just wander if it will work for those Vox circuits if I use 2N3904 & 2N3906 trannies. Will I have to create another bias circuit or will it work just plugging the 12v into the supply points as shown?

Thanks in advance.

RDV

[R.G.]

The first preamp stages run on +20, the mixer and limiter run on +16-17.

One of the things that eats my time up when I don't have anything else to do... ?!?!? is I'm writing a service manual for Thomas Vox solid state amps. I have a lot of data, enough to drown in, and getting it into book form is slow going. I have no illusion that I'll make any money from this, but I like the amps and hate to see them junked.

[Doug_H]

That's a cool looking preamp, Ricky. I wonder how it would sound as an outboard pedal?!? Hmmm...

Doug

[RDV]

That's a cool looking preamp, Ricky. I wonder how it would sound as an outboard pedal?!? Hmmm...

Doug

Dunno. Trying to figure out if it will run on stompbox voltage.

RDV

[RDV]

Well it doesn't work at stompbox voltage!

RDV

[Doug_H]

To get it to run at 9v you would have to re-bias it. Another idea might be to use a charge pump IC if it doesn't draw too much current.

I just realized something... That thing looks like a bootstrapped Darlington pair. Look at the npn boost and compare. I guess I should read the GEO article, I'm sure it already mentions that. Anyway, it makes me wonder what it would sound like to put all that fancy-schmancy tone circuitry on the output of an npn boost. :?:

Doug

[R.G.]

It is indeed a bootstrapped darlington, for high input impedance. The tone controls are a reasonably simple mod to the stock amplifier tone stack, but scaled to lower impedance and with the inductor thrown in for a midrange boost - a fixed frequency wah setting.

[RDV]

Boy! I am rusty. This thing works well at stompbox voltage. I made the 100 ohm to ground another 100k. DOH!!

I've only built the preamp part on perf so far, but I must say that just this part with a volume pot tagged on the end would make a cool treble booster. It reaaallly is bright. I guess I'll move on to the EQ section and see what happens.

Stay Tuned.

RDV

[Eric H]

The first preamp stages run on +20, the mixer and limiter run on +16-17.

The ampage schematic shows the limiter at 18v single-supply  --is it incorrect?

I remember your old mystery-circuit postings, but find it more interesting now.

-Eric

[RDV]

I finished the EQ section and really like it. This would be a good stomp box. I still need to get a 3-way rotary switch for the mid-boost. I may make a second one for a stompbox version. It has a very Voxy sound.

RDV

[RDV]

The first preamp stages run on +20, the mixer and limiter run on +16-17.

The ampage schematic shows the limiter at 18v single-supply  --is it incorrect?

I remember your old mystery-circuit postings, but find it more interesting now.

-Eric

Since these things work on a single supply, I would think that almost any voltage will work with a higher voltage working better.

RDV

[Eric H]

The first preamp stages run on +20, the mixer and limiter run on +16-17.

The ampage schematic shows the limiter at 18v single-supply  --is it incorrect?

I remember your old mystery-circuit postings, but find it more interesting now.

-Eric

Since these things work on a single supply, I would think that almost any voltage will work with a higher voltage working better.

RDV

If you look carefully at RG's post, Ricky, He says it's plus-16v/ minus17v

--at least that's how I read it. IOW, a dual-supply

-Eric

[RDV]

If you look carefully at RG's post, Ricky, He says it's plus-16v/ minus17v

--at least that's how I read it. IOW, a dual-supply

-Eric

He's refering to range of voltage rather than a dual supply. There is a dual supply in those Vox SS amps but I believe that the dual part is used in the power amp sections only.

Chime in if I'm wrong R.G.

RDV

[R.G.]

You're right Rick. It was 16 to 17 volts, etc.

Sorry - I wasn't clear. I do try to write bipolar voltages as "+/-" and I hope I'm consistent.

[RDV]

I'm having a little trouble finding a 3-way switch. I guess I'll get the Rat Shack 6-way and put 6 caps on the mid boost! I really like the .22uF one, it's nice and thick!

RDV

[Steben]

euhm... won't those 820 and 100 ohm resistors suck enormous amounts of current for a stompbox?
Will it be easy to rebias it towards 8k2 and 1k for example?

[Doug_H]

euhm... won't those 820 and 100 ohm resistors suck enormous amounts of current for a stompbox?
Will it be easy to rebias it towards 8k2 and 1k for example?

I was wondering about that too.  You would need to adjust the 100k/33k series string that sets the reference voltage for the base if you changed the collector-emitter biasing.  But darlington pairs are used to provide really high current gain, so the "high current biasing" may be needed to accomodate that.

Something else I noticed were the back-to-back polarized electros (essentially non-polarized). If the individual cap values are "2" and "1", then you could easily sub those with 1u and .47u film caps, respectively.

Doug

[Doug_H]

I'm having a little trouble finding a 3-way switch. I guess I'll get the Rat Shack 6-way and put 6 caps on the mid boost! I really like the .22uF one, it's nice and thick!

RDV

Sounds cool! Should be interesting to hear how the limiter works out.

Doug

[RDV]

euhm... won't those 820 and 100 ohm resistors suck enormous amounts of current for a stompbox?
Will it be easy to rebias it towards 8k2 and 1k for example?

The version I'm building now is for an amp, I haven't started a stompbox version yet. I'm testing it with a very strong 9v adapter(about 850ma).

But you're right.

RDV

[changes]

guys have I missed something?
Is this a vox top boost preamp schem?
How are you gonna use it?
And why is the limiter there?
If I get it right it should go in this order: vox preamp+limiter+power amp
What type of trannies and diodes should I use?